Characteristics of Relaxation Process in an Optically Coupled Quantum-Dots System

Mauricio Garrido

Yoshihiro Nambu, Akihisa Tomita, Hideaki Saito Hideaki Saito +1 more

Spectral broadening and cross relaxation of an aggregate of highly carrier-injected quantum dots have been studied experimentally by evaluating the gain saturation and optical-wave mixing characteristics of traveling-wave laser amplifiers that include self-assembled 0.98-µm InGaAs dots. Measured signal-gain spectra at room temperature showed that the gain decreases over the entire gain spectra when an intense and narrow-linewidth saturating signal is injected at a higher-order subband transition of the quantum dot. Differential-gain spectrum was calculated from the saturated and unsaturated signal gain spectra, and revealed asymmetric gain saturation with respect to the wavelength of the saturating signal. These results suggest that cross relaxation is so fast that the dot aggregate has a homogeneously broadened gain spectrum. Self-saturation and cross-saturation characteristics, and the detuning characteristics of highly nondegenerate four-wave mixing were also investigated. The latter revealed that the characteristic times of two cross-relaxation processes are about 1 ps and 100 fs.

Aleksandar Janković, R. Ferreira, G. Bastard

We present a model which treats non-perturbatively the effects of both dipolar coupling and reservoir fluctuations on the interband optical response of a single quantum dot. We are able to study the transition to a Rabi-like situation with increasing excitation power. The incorporation of an energy relaxation process (for instance, due to polaron relaxation among the dot polaron levels) in a three-level system allows us to simulate both photoluminescence (PL) and photoluminescence excitation experiments. The PL spectrum displays one or a few lines depending on the excitation intensity, relaxation efficiency and reservoir fluctuation rates.

E. Cota, F. Rojas, Sergio E. Ulloa

Duncan G. Steel

Gabriela M. Slavcheva

Abstract The optically‐induced coherent spin dynamics of a single spin confined in a charged quantum dot (QD) is theoretically studied employing coupled vector Maxwell‐pseudospin formalism. Generalized pseudospin master equation is derived for description of the time evolution of spin coherences and spin populations including spin population transfer and dissipation in the system through spin relaxation processes. The equation is solved in the time domain self‐consistently with the vector Maxwell equations for the optical wave propagation coupled to it via macroscopic medium polarisation. Using the model the long‐lived electron spin coherence left behind a single resonant ultrashort optical excitation of the electron‐trion transition in a charged QD is simulated in the low‐ and high‐intensity Rabi oscillations regime. Signatures of the polarised photoluminescence (PL), predicted by the model, such as the appearance of a second echo pulse after the excitation and characteristic PL trace shape, are discussed for realization of high‐fidelity schemes for coherent readout of a single spin polarisation state. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)